The issue before us is whether a subpoena duces tecum served on a non-party Tribe and seeking documents relevant to a civil suit in federal court is itself a “suit” against the Tribe triggering tribal sovereign immunity. Exercising jurisdiction under 28 U.S.C. § 1291, pursuant to the collateral order doctrine, we hold the answer is yes. We therefore reverse the district court’s denial of the Tribe’s motion to quash based on tribal immunity.